The crop harvested from the tea plant, Camellia sinensis, comprises young leaf (green leaf) shoots which consist essentially of two or more leaves and the unopened terminal leaf bud or tip. This crop when suitably processed results in the various types of tea of commerce. For instance, black leaf tea is generally made by subjecting the plucked leaves (green leaf) to a series of traditional processing steps including (1) withering, (2) disintegration of the leaves by rolling, macerating or otherwise bruising the withered leaf to release the juices and enzymes, (3) fermentation (enzymatic oxidation) to develop the characteristic color and flavor, the latter including taste and aroma, of black tea, and (4) firing or drying, to deactivate the enzymes which catalyze fermentation. The extent of the fermentation varies in commercial practice, from black to various gradations between green and black. Partially fermented tea is known as "oolong" tea. An infusion of dried black leaf in hot water provides a liquor having a color ranging from gray to golden to brown to rosy pink in hue and exhibiting various degrees of brightness. It is generally considered that a bright orange-red or red color is most desirable, but this depends on the intended market for the tea. It is known that the color and flavor of a black tea infusion are influenced by the nature of the aforesaid processing steps. Another factor which also affects the color and flavor is the nature of the original green leaf from which the tea is manufactured, as determined by the type of plant locality and conditions under which the tea is grown. Factors such as the country of origin, soil conditions, rainfall and other climatic variations all have a bearing on the quality of the tea, and variations may occur not only from one locality to another, but also from one season to another for tea grown at the same location.
Similar considerations apply to the quality of the various instant black teas which conventionally are manufactured by extracting the water-soluble ingredients produced during black leaf tea manufacture and forming by known techniques hot or cold water-soluble concentrated tea extracts or tea powders. Optionally, it is known to produce such extracts or powders from green tea extract by slurry fermentation or by non-enzymatic chemical conversion. Again, because of variability of the starting tea and the nature of the processing, the control of tea color and flavor is difficult. The determination of quality of the tea tends to be a subjective assessment by the tea taster. From these considerations, it will be appreciated that there is a long-felt need for being able conveniently to modify the organoleptic qualities of black tea, particularly from the standpoint of enhancing the color and flavor attributes thereof.
As used in the present application, the term "tea" is intended to mean leaf tea (unless otherwise specified, black or oolong), aqueous extracts and infusions of tea and tea compositions produced therefrom in either liquid or powder form and which are hot water-soluble or cold water-soluble and fractions thereof.
It is known that the color of an infusion of black tea is provided by the oxidation of polyphenols. These colored oxidation products may be divided into two classes, theaflavins and thearubigins. The former may be separated from the latter by extraction of an aqueous extract of tea with ethyl acetate, the theaflavins being extracted into the ethyl acetate. Characteristically, the theaflavins impart a yellowish color to a tea infusion which may approach a bright orange-red color in concentrated solution, whereas thearubigins impart a reddish-brown color. The total color of the tea is related to the sum of the theaflavin and thearubigin contents and, hence, to strength, whereas brightness is primarily an expression of the proportion of the color due to the theaflavins. In a given black tea infusion, if the thearubigin value is high and the theaflavin value low, the tea is considered dull. If the levels of both the theaflavins and thearubigins are low, then the tea is thin and gray. To achieve the normally preferred bright orange-red or red color, it is believed that the tea should contain an average thearubigin content and a high theaflavin content.
It is also known that the tea plant is an aluminum accumulator and the aluminum content in the plucked leaves from the naturally occurring tea plants is generally within the range of from 200 to 1000 ppm by weight (dry basis) as compared with 50 to 100 ppm in other plant species. The stated aluminum content is normally also present in black and oolong teas. The role of aluminum in the metabolism of the tea plant has been a matter of some speculation without resolution. The effect of variations in the amount of aluminum accumulated in the leaf on tea infusions has not been known.